1. Field of the Invention
The present invention relates to a water-soluble core for pressure die casting and a process for producing the core. More particularly, the invention relates to a water-soluble core for pressure die casting having a desired strength prepared by compression molding a granular water-soluble salt.
2. Description of the Prior Art
In case of casting metallic castings containing cavities by normal pressure or low pressure casting, such as sand mold casting, gravity die casting, low pressure die casting, etc., a casting having complicated cavities has hitherto been readily produced using a known sand core prepared by bonding sand grains with a caking agent, such as a synthetic resin. However, in the case of casting a metallic casting containing cavities in a pressure die casting method in which a molten metal such as an aluminum alloy, a zinc alloy, etc., is poured in a precise metallic die under a high pressure of a few hundreds kg./cm..sup.2 or higher, and cores other than those having high strength and capable of being readily removed after casting are unsuitable in this method. Various cores for pressure die casting having such properties have hitherto been proposed but they each have difficulties and the cores which can be satisfactorily used for the purpose, have hitherto not been available.
For example, there is known a casting method in which a core of a Zn-base alloy is used as the core for die casting aluminum or aluminum alloy and after casting the core is melted away by heating, but is such a method, the core is partially washed away by pouring the molten metal and furthermore, the complete removal of the core is difficult. Also, it is known to use a water-soluble core prepared by melting and molding a mixture of sodium metasilicate (Na.sub.2 O.SiO.sub.2) and sodium disilicate (Na.sub.2 O.2SiO.sub.2) but the use of the core is accompanied with the flaw that at the dissolution of the core, its aqueous solution is brought into a strongly alkaline state and erodes aluminum, zinc, etc. Furthermore, a water-soluble core prepared by melting and molding sodium chloride or calcium chloride is known, but a core made of sodium chloride will shrink during molding and the production of such a core having a high dimensional precision is quite difficult. A core made of calcium chloride is subject to the characteristics of a high hygroscopic property such that the core after molding is required to be maintained at a temperature of at least 200.degree. C. before being used for casting.
Still further, a method of producing a salt-type core by compression-molding a sodium chloride or potassium chloride powder at a pressure of 300-500 kg./cm..sup.2 and then sintering the molding at 500.degree.-750.degree. C. for 0.5-2 hours is disclosed in the specification of the Anderko U.S. Pat. No. 3,356,129, but the core shown in the Anderko patent is for a gravity die casting of pistons, etc., and has low strength, which makes the core unapplicable for pressure die casting. That is, in a casting method requiring a core of high precision, such as pressure die casting, the core produced by sintering a salt at a high temperature between 500.degree.-750.degree. C. for a long period of time as in the aforesaid method, cannot be applied since the dimensional precision of the core is greatly reduced at the production thereof besides the high production cost thereof.